This invention concerns a method for the synthesis of conjugated polymers. In particular the invention concerns a method for producing photoluminescent or electroluminescent conjugated polymers for use in photoluminescent and electroluminescent devices.
Conjugated polymers and in particular poly(arylene vinylene)s (PAVs) have attracted increasing interest in recent years as potential electroluminescent and photoluminescent materials. The synthesis of such polymers has thus been the focus of much research.
A common synthetic approach in forming poly(arylene vinylenes) employs the Wessling reaction. In this reaction, a polymer is synthesised which contains arylene units, vinylene units being introduced via an elimination reaction. Such a route is capable of a small degree of selectivity, but good control over vinylene chemistry is not possible in routes which rely on elimination reactions. A route which allows the introduction of some cis vinylene into the polymer backbone is described in Science Vol. 269, page 376 (1995).
A method suitable for the synthesis of soluble poly(arylene vinylenes) is the McMurry reaction. The first effective use of this reaction in the synthesis of PAVs was reported by Feast and Millichamp in Polymer Communication, 1983, 24, 102. PAVs with a polymerisation degree of more than 100 can be produced by the reductive coupling of dialdehydes and diketones.
More recently, aryl boronic acids have been coupled with aryl halides (catalysed by palladium phosphines and sodium carbonate) to produce PAVs (Akta chemica Scandinavia 1993, 47, 221; Macromol Rapid Commun. 1996, 17, 239; Macromolecules 1996, 20, 1082; and Polymer 1989, 30, 1060). This condensation polymerisation route is termed the Suzuki reaction and proceeds under mild conditions tolerating a varied range of functional groups. The Polymers produced show a degree of polymerisation of 100 or more.
A further route to PAVs is a coupling reaction between Grignard reagents and aryl halides, catalysed by nickel compounds. This reaction, termed the Yamamoto reaction, was applied to the preparation of unsubstituted poly-p-phenylene type polymers for the first time by Yamamoto et al in Bull Chem. Soc. Jpn., 1978, 51, 2091. This particular reaction has been limited to the synthesis of poly-p-phenylene and has not yet been reported as a potential route to PAVs.
The inventors identified a problem in all of the above prior art methods, in that none of the methods allowed for reliable control of the cis/trans vinylene ratio in the polymer products.
Accordingly, the present invention provides a method for synthesizing a poly(arylene vinylene), which method comprises selecting a vinyl monomer by controlling the ratio of cis isomer to trans isomer and forming the poly(arylene vinylene) from the vinyl monomer, such that the desired cis vinylene to trans vinylene ratio is obtained in the poly(arylene vinylene) product.
The present invention also provides a poly(arylene vinylene) obtainable according to the above method, light emitting electronic components or devices comprising the poly(arylene vinylene) and an optical component or device comprising the poly(arylene vinylene). In the context of the present invention, a poly(arylene vinylene) is an oligomer or a polymer which contains in its backbone at least one arylene unit and at least one vinylene unit.
An advantage of the present invention is that the present synthesis allows control of the cis/trans ratio of the vinylene units in the poly(arylene vinylene) products. The invention also makes available poly(arylene vinylene)s having improved electroluminescent and photoluminescent properties, these properties being controlled by varying both the cis/trans vinylene ratio in the polymer, and the vinylene substituents of the polymers.
A further advantage is that the number of in-chain phenylene rings in the repeat units can be varied using the present method, whilst maintaining the solubility of the polymer. In addition, the larger the number of in-chain phenylene rings, the more similar in structure and properties is the polymer to those of poly(p-phenylene), which has been shown to emit in the blue region of the spectrum. By controlling the cis and trans vinylene ratio in the polymer chains it is possible to control the properties of the resultant polymers, particularly the electro-optical properties.
Any vinyl monomer can be used in the method of the present invention, provided that the ratio of the cis isomer to trans isomer in the vinyl monomer can be controlled.
Control of the ratio of cis isomer to trans isomer can be carried out by any means which in turn allows control of the ratio of cis to trans vinylene units in the polymer product. When controlling the ratio of cis isomer to trans isomer in a monomer containing a mixture of both isomers, successive crystallisation is preferably used to select a desired isomer ratio. Alternatively, if both isomers are available in pure form, the correct cis/trans isomer ratio is preferably selected by mixing. Where the vinyl monomer is to be synthesised, the cis/trans ratio is preferably controlled by selecting the appropriate reaction conditions and/or reaction catalyst, and the appropriate purification techniques such as fractionation, distillation, recrystallisation and the various types of chromatography.
In the present invention, the poly(arylene vinylene) is preferably produced via the Yamamoto reaction or the Suzuki reaction.
The Yamamoto reaction is a coupling reaction between Grignard reagents and halides, catalysed by nickel compounds. In the case of coupling aryl groups the reaction can be summarised according to the following reaction scheme: 
Ar and Arxe2x80x2 can be the same or different. X is a halide group, preferably a bromide group. In the context of the present invention, either or both of the Ar and Arxe2x80x2 groups must contain a vinyl group so that the final product is a poly(arylene vinylene). Only one of the monomers in the above scheme need contain an aryl group to produce a PAV, but it is preferred that both monomers contain an aryl group.
The Suzuki reaction is carried out by coupling a dihalide with a boronic acid derivative. The reaction is preferably carried out in the presence of sodium carbonate and is preferably catalysed by a palladium phosphine complex. In the case of coupling aryl groups, the reaction can be represented schematically as follows: 
Ar and Arxe2x80x2 can be the same or different. X is a halide group, preferably a bromide group. In the context of the present reaction either or both of Ar and Arxe2x80x2 must contain a vinyl group, so that a PAV is produced. Only one of the monomers in the above scheme need contain an aryl group to produce a PAV, but it is preferred that both monomers contain an aryl group.
When the Suzuki reaction or the Yamamoto reaction is employed in the method of the present invention, it is preferred that the vinyl monomer is a dibromide but other dihalides can be used. Thus, dichloride and diiodide vinyl monomers may also be employed. If the vinyl monomer contains an aryl group then this can be polymerised in the absence of further monomers to form a poly(arylene vinylene). However, it is preferred in the present invention that the poly(arylene vinylene) is formed by co-polymerising the vinyl monomer with a further monomer. If the vinyl monomer contains aryl groups then the further monomer need not be an aryl monomer. However, it is preferred in the present invention that the further monomer is an aryl monomer, such as a phenyl or biphenyl derivative.
Particularly preferred vinyl monomers for use in the present invention include 1,2-diphenyl-1,2-di(4-bromophenyl)-ethene, or 1,2-dimethyl-1,2-di(4-bromophenyl)-ethene.
In the case where the vinyl monomers themselves are to be synthesised, it is preferred that they are produced by the McMurry reaction in which a diketone or dialdehyde undergoes reductive coupling to produce a vinyl compound.
The present invention allows not only for the control of the cis/trans ratio of the vinylene units in the polymer products, but also the molecular weight of the polymers produced. It has been found that the molecular weight of the polymers is dependent upon the ratio of cis isomer to trans isomer in the vinyl starting materials as well as the solvent system which is used for polymerisation. A cis isomer to trans isomer ratio of approximately 1:1 has been found to give rise to polymers having the largest molecular weight. Similarly, solvents in which the polymers are more soluble are more likely to give rise to high molecular weight products. Accordingly, when the Suzuki reaction is employed in the present method, it is preferred that the reaction takes place in a solvent comprising THF in the presence of aqueous potassium carbonate.
The cis isomer to trans isomer ratio in the vinyl monomer is preferably from 80:20 to 20:80, more preferably 70:30 to 30:70, more preferably still 60:40 to 40:60 and most preferably 45:55 to 55:45.
Inevitably, polymers having a range of molecular weights are produced in any one polymerisation reaction. A poly(arylene vinylene) having the desired molecular weight can be isolated from the reaction products by fractionation.
The molecular weight of the poly(arylene vinylene) products can be affected by the duration of the reaction. In general it is preferred that the reaction is allowed to proceed for 48 hours or more, and in some cases 96 hours or more.
The PAVs produced using the method of the present invention are preferably photoluminescent and more preferably also electroluminescent. It is preferred that the photoluminescence efficiency of the PAVs is, in increasing order of preference, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more or 70% or more. It is also preferred that the electroluminescence efficiency is high, e.g. 20% or more.